Many differential global position corrections networks exist, including FAA WAAS, OmniStar, JPL, USCG Beacons, local commercial networks, local community cooperatives networks, virtual reference station networks and local government funded networks. Differential corrections provide more accurate position determinations.
In wide area networks (e.g., WAAS, OmniStar, and Starfire), geosynchronous satellites provide differential correction information. The footprint of the beams from the geosynchronous satellites is large, resulting in a wide area network. A relative few, such as 10-30, reference stations are provided in any footprint of the geosynchronous satellites for hemisphere corrections. The reference stations send phase measurements on landlines to a central facility. The central facility provides the differential corrections to the geosynchronous satellites for beaming to mobile devices. Mobile receivers determine location with differential based accuracy from global positioning satellite (GPS) signals and the differential corrections. The closest reference station may be over 100 miles away from the mobile receiver.
For local area systems (e.g., USCG Beacons), terrestrial based communications provide differential corrections. One or more local reference stations transmit differential corrections on a radio transmitter or modem. Receivers within the range of the radio transmitter or modem calculate position based on the differential corrections received from the reference stations. Carrier phase based position determinations may provide about 2 or better inch accuracy. However, local area systems may not be generally available, may be more likely subject to interference from terrestrial structures and may be more costly to use than wide area networks.
Some local area systems provide wide area network position determination as a back up. Positions of the mobile receiver are continuously determined from local area differential corrections until failure of the local area network. For example, the mobile receiver uses the local area network until a building, hill or other structure blocks transmission of the local area differential information. Once a failure occurs, the mobile receiver uses the wide area network until the local area network is again available.